travis: Roll back to Trusty as Xenial support is not stable yet
travis: Roll back to Trusty as Xenial support is not stable yet

dreal/dreal4,dreal/dreal4,soonho-tri/dreal4,dreal/dreal4,soonho-tri/dreal4,soonho-tri/dreal4,soonho-tri/dreal4,dreal/dreal4